Fiber Optic Sensor

ABSTRACT

3-Mirror fiber optic sensor for simultaneous measurement of dual parameters such as hydrogen concentration and temperature

CLAIM FOR BENEFITS OF AN EARLIER FILING DATE, UNDER 35 U.S.C. 120

Applicant claims the benefit of Provisional Application 60/547,163, filed Feb. 24, 2004. Attorney docket 82005

FIELD OF INVENTION

This invention is in the field of transducers using light and fiber optics for qualitative and quantities measurements.

BACKGROUND OF THE INVENTION

Fiber optics and light have been used to measure hazardous materials like hydrogen gas, where a safe and accurate detection is essential. Because fiber optic technology employs non-reactive light guided inside a fiber, permitting remote, real-time measurement implementation, it can be used in explosive/hazardous environments such as hydrogen leak detection.

Other current, commercially available technologies used to measure hydrogen concentrations are primarily electrical in nature. Some examples are: Hydrogen Field Effect Transistors (HFETs) such as Metal Oxide Semiconductor (MOS), Metal Insulator Semiconductor (MIS) Field Effect Transistors and semiconductor sensors²⁰⁻²² such as thin film resistors. Electrical means of detection are inherently dangerous because of the fact that they use electrical current near a potentially explosive environment to detect hydrogen. Furthermore, they cease to function as intended at cryogenic temperatures.

Conventional Extrinsic Fabry-Perot fiber optic sensors based on the interference of the two or more light waves from separate multiple light sources that are reflected from the two reflective surfaces at the end of the Fabry-Perot cavity can be used to measure various measurands such as strain, force, temperature, pressure, etc. The measurand effect on the sensor changes the cavity length, producing a change in the phase of the reflected light and the interference patterns of the reflected light as an interferometer output signal. The output signal is a sinusoid generated by the two reflected beams moving in and out of phase with respect to one another. The number of sinusoidal cycles (fringes) in the output signal is proportional to the change in the length of the cavity for a given wavelength of light. Since the change in length of the cavity is related to the measurand, the number of fringes produced can be correlated to the parameter measured, such as temperature.

The conventional sensor described above requires that the temperature of the sensing point be held constant if another parameter is being measured. This requires heating or cooling of the sensing point, for example, the temperature can be maintained by means of a relatively high power laser. Such applications require separate monitoring of the temperature at the sensing point. Where two separate independent variable parameters are measured, for example temperature and hydrogen, a temperature sensor probe is used with the hydrogen sensor probe. When the two independent variables need to be measured by a single sensor, two wavelengths (i.e., an additional light source) must be employed. This is the case where temperature affects the gap length due to expansion and contraction of the materials. In addition to the added cost and complexity, the dual or multiple wavelength method is usable only over a limited range before the two variables cannot be separated (least common multiple of the wavelengths).

These disadvantages of conventional systems and methods for measurement using electricity or light are overcome by the invention described in this application, as would be understood by those skilled in the art by reading the following description of the invention.

SUMMARY OF THE INVENTION

This invention, is as shown by the disclosed inventive principles, for a preferred embodiment which does not limit or restrict the use or modification of the disclosed invention, as would be apparent to those skilled in the art.

The disclosed invention overcomes the disadvantages of conventional fiber optic or electrical measurement by a system and method of using a 3-mirror sensor that measures two or more independent variable parameters substantially at one location and substantially simultaneously, by the use of a single wavelength or light source. In a conventional two cavity arrangement, there exist 4 reflective surfaces. In the case of the disclosed invention, the two cavities have a shared common reflective transmissive surface thereby resulting in 3 reflective surfaces. An example as used to explain the invention would be hydrogen concentration and temperature. The 3-mirror sensor has the ability to extract hydrogen concentration and temperature, at substantially the same location or measurement point of interest, simultaneously with a single sensor head and by use of a single wavelength, thereby minimizing cost and complexity and increasing accuracy.

An improvement in accuracy is achieved, as would be recognized by those skilled in the art, by reducing the system and method to the measurement of a plurality of variable parameters, which are located at substantially the same point, using a single wavelength or light source and with the measurement accomplished substantially simultaneously.

The invention, as disclosed in a preferred embodiment, uses a single sensor to illuminate or energize a plurality of optical cavities, to measure a plurality of variables. As disclosed in a preferred embodiment, where temperature and hydrogen concentration is measured, two optical cavities with three reflective surfaces within a single sensor using one optical wavelength can be used.

As disclosed in a preferred embodiment, the first of the two optical cavities is made sensitive to a first parameter, as shown in a preferred embodiment, to temperature. As disclosed in a preferred embodiment, the second of two optical cavities is sensitive to hydrogen and temperature. For example, as disclosed in a preferred embodiment, Palladium (Pd) or some alloy that includes palladium may be used as a metal that is sensitive to hydrogen. Since the rate of change of volume of a cavity made with a hydrogen sensitive metal, for example Palladium, is different than the rate of change of volume with temperature, the relative phases of the light output signals from the two optical cavities, changes in response to the separate effects to the two separate variables measured. As would be understood by those skilled in the art, for a temperature sensor, the rate of change of volume with temperature would be different from the rate of change of volume for a sensor responsive to hydrogen, and the light output signal from the sensors' optical cavities will be fringes of one frequency being modulated by an envelope of a different frequency. The signals produced having different frequencies, separately indicative of temperature and hydrogen, for example, can be separated and the separate effects or presence, of temperature and hydrogen on the sensor can be separately detected.

As shown, a plurality of independent variables substantially at one location can be measured substantially simultaneously, with one sensor and one wavelength.

As would be understood by those skilled in the art, from reading the Detailed Description, following, the disclosed method and system can be used in applications that require the monitoring of other independent variables. For example: pressure-temperature, strain-temperature, chemical and biological sensing, to name a few examples.

One example of an application of the disclosed inventive principles, without limiting the applications of the disclosed inventive principles, is in the fuel cell energy systems. At this time, it is estimated fuel cell technology soon will be 34% of the energy market and will play an important role in the energy economy in stationary fuel cells for power generation and mobile fuel cells in the transportation sector. A cost effective and accurate sensor, for measuring hydrogen, as disclosed herein according to the inventive principles for a preferred embodiment, using a single interferometer sensor, will be needed to detect hydrogen leaks and monitor hydrogen usage.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shown in schematic form, a preferred embodiment of the fiber optic sensor in cross section.

FIGS. 2 a and 2 b, shown in juxtaposition, the fringe rates of the reflected light output of the fiber optic sensor of FIG. 1, for a fringe rate A responsive to temperature at Cavity 2, a fringe rate B responsive to temperature at Cavity 1, and a fringe rate C responsive to the presence of hydrogen at Cavity 2.

FIG. 3, shows for a preferred embodiment, a system for detecting the reflected light output of the fiber optic sensor as shown in FIG. 1 and the fringe rates as shown in FIG. 2 a and 2 b.

FIG. 4, shows for a preferred embodiment, a part of the system of FIG. 3, for filtering the fringe rates.

DETAILED DESCRIPTION

The invention is described with reference to the disclosed inventive principles, as shown for a preferred embodiment, in the cross section view of FIG. 1. The sensor is disclosed as comprising two optical cavities, Cavity 1 with an optical length d1 and Cavity 2, with an optical length d2. As would be understood by those skilled in the art, the respective optical lengths d1 and d2 are formed between the partially reflective, partially transmissive ends of Fiber Lead 1 and Fiber Lead 2. Reflective Surfaces R1, R2, are partially transmissive and partially reflective surfaces reflecting light from a laser light source directed to the sensor from Point X. The laser light source, as described for a preferred embodiment is not shown as its application would be understood by those skilled in the art.

As would be understood by those skilled in the art, the disclosed invention is not limited to a laser light of electromagnetic radiation, but may use any optical source with sufficient coherence consistent with the disclosed inventive principles.

As shown in a preferred embodiment, Reflective Surface R1, produces a reflection of the light from a single source or at a single wavelength, entering Fiber Lead 1, from Point X, as shown by path E1, and transmits light through Cavity 1, to Fiber Lead 2, where it is reflected at Reflective Surface R2, as shown by path E2. Similarly, the light from Point X, transmitted to Fiber Lead 1, is transmitted through Cavity 1 and Fiber Lead 2 to Cavity 2, formed within Fiber Lead 2, and to Reflective Surface R3, where it is reflected as shown by path E3. As seen in FIG. 1, for a preferred embodiment, Cavity 1 is between Reflecting Surface R1 of Fiber Lead 1 and R2 of Fiber Lead 2 and Cavity 2, is between Reflecting Surfaces R2 of Fiber Lead 2 and R3 of Fiber Lead 2.

Fiber Leads 1 and 2, forming Cavity 1 is made sensitive to temperature as described in the following. Cavity 1, is fixed by epoxy, for example, to a temperature sensitive medium, as shown in a preferred embodiment to an aluminum plate, and made from a material substantially neutral to the temperature variable parameters in measurement, for example, a glass tube. The glass tube may be replaced by any other suitable material, where a different measurand is of interest or where the sensitivity of the glass tube to the temperature in measurement may be factored into the final temperature measurement. By measurand is meant the variable parameter of interest and measured.

Cavity 2, for the hydrogen related measurement, as shown and disclosed in a preferred embodiment, is formed within Fiber Lead 2, with Reflective Surface R3, held or mounted within the Palladium tube. For a preferred embodiment, Palladium is used for its sensitivity to hydrogen. However, as would be known and understood by those skilled in the art, other suitable materials may be used as may be sensitive and suitable or measuring other independent variables and this disclosed inventive principles are not limited to the measurement of hydrogen or temperature.

For a preferred embodiment, Cavity 1 and Cavity 2, are mounted on a common aluminum plate, and are sensitive to temperature. Cavity 2, mounted in the hydrogen sensitive Palladium tube, is made sensitive to hydrogen. As would be known to those skilled in the art, palladium changes in its dimension when exposed to hydrogen. An induced change in the dimension of the Palladium tube will cause a change in the length of Fiber Lead 2, without any substantial effect on Fiber Lead 1, separated from the Palladium tube by Cavity 1, within the substantially temperature neutral glass tube. The change in the length of Fiber Lead 2 produces a change in Cavity length d2, and a change in relative phase of the signal reflected in path E3, relative to the light reflected in path E2, within intrinsic Cavity 2 and the interference pattern established in extrinsic Cavity 1 by the relative phase of the light reflected in paths E1 and E2.

As would be understood by those skilled in the art, sΔystem disclosed in FIG. 1, according to the disclosed inventive principles as shown for a preferred embodiment, is a three mirror interferometer, using a single light wavelength or light from a single source and measuring a plurality of independent variables substantially at one spatial location.

For example, for a material with initial length L₀ and coefficient of linear expansion α_(A), that undergoes a temperature change of ΔT=T₁-T₂, the change in length (ΔL) is given by the formula ΔL=L _(o)*α_(A) *ΔT   Eq. 3.1

For two different materials of length L₁ and L₂ with coefficient of linear expansion α₁, and α₂ (α₁>α₂) and arranged as the dual cavity interferometer, the change in length of the material attached to these materials is, ΔL=[(L ₁*α₁)−(L ₂*α₂)]*ΔT   Eq. 3.2

The change in relative length, of the Fiber Leads 1 and 2 therefore, produces a change in light path E1, E2, and E3 and the light reflected at Reflective surfaces R2 and R3, in respective light paths E2 and E3, will be seen to have different phase relationships with respect to one another and E1. This allows the three waves to interfere and the resulting interference output signal or interferometer output signal, is proportional to the resultant optical power at the detector, P₀. P_(o)α(E₁+E₂+E₃)²   Eq. 3.3 Where; E₁˜E₀ cos [f(θ_(o))]  Eq. 3.4 E₂˜E₀ cos [(g(θ_(o)ΔT)]  Eq. 3.4 E₃˜E₀ cos [(h(θ_(o)ΔT,ΔH₂)]  Eq. 3.4 And E₀ is a constant

-   -   E₁ is the Electric field reflected back from R₁     -   E₂ is the Electric field reflected back from R₂     -   E₃ is the Electric field reflected back from R₃     -   fθ_(o) is the initial phase     -   g(θ_(o)ΔT) is function of the initial phase and change in         temperature     -   (h(θ_(o) ΔT,ΔH₂) is a function of the initial phase, change in         temperature and hydrogen

For a preferred embodiment, FIG. 2 a shows two fringe rates that are caused by temperature alone. These are denoted as the slowly changing envelope A (units of fringes per hour) for changes in temperature at the second Cavity C2 and the more rapidly changing B (units of fringes per minutes), at the first cavity C1.

In a preferred embodiment, hydrogen only affects the second cavity, Cavity C2, shown as a palladium tube, and does not affect the temperature sensitive cavity C1. The effect of hydrogen would be to increase the length of the palladium tube and cause the length of the second cavity, Cavity 2, to decrease by this amount. This causes the phase relationship of E3 to change with respect to E1 and E2 thereby causing the three fields of reflected light from Reflective Surfaces R1, R2, and R3, to interfere, in response to the change in the two independent variables, shown in a preferred embodiment, as temperature and hydrogen. In a preferred embodiment, the system response due to the effect of hydrogen on the fiber optic sensor is shown in FIG. 2 b. The fringe rate due to hydrogen is denoted as C and has units of fringes per second. From FIG. 2 b, it can be seen that the fringe rate due to hydrogen C, is significantly larger than temperature related fringe rates.

Since the fringe rate shown as A due to temperature at Cavity 2, and B due to temperature at Cavity 1, and the fringe rate C at Cavity 2, due to hydrogen, are different sufficiently to be separately detectable, the temperature effect can be separated from the hydrogen effect by frequency based signal processing, as would be known to those skilled in the art and as explained below.

As shown for a preferred embodiment in FIG. 3, a fiber optic coupler, shown for example in FIG. 3, as FO Coupler is arranged to receive the interferometer output signal produced by the phase relation of the reflected in light paths E1, E2, and E3, from Reflecting Surfaces R1, R2, R3, and transmit it to an optical to electronic signal detector, as would be known to those skilled in the art.

As shown in preferred embodiment, the fringe rate optical signals due to the presence of hydrogen and the effect of temperature are different, fringe rate A indicative of the fringe rate at the second cavity Cavity, 2, due to temperature being slow in relation to the fringe rate B due to temperature at the first cavity C1 and the higher fringe rate C due to hydrogen, at the second cavity C2.

As shown in the block diagram of FIG. 4, the electrical signal from the detector, as shown in FIG. 3, may be processed to filter or separate the signals indicative of fringe rate B and the temperature and fringe rate C and of hydrogen.

As would be known to those skilled in the art, signal processing using software techniques involves operating on the interferomertic output signal using fourier transform methods may be uses to separately identify the high and low frequency components that result from such an operation are identified in FIG. 4, as B and C.

As may be seen and understood from the disclosed inventive principles, signal processing will filter out the low frequency component of the optical output from the interferometer output signal, thereby eliminating the temperature effect from the hydrogen responsive signal. The low pass filter will filter out the high frequency component of the output from the interferometer sensor, leaving the temperature signal. Thus by processing the signal for the fringe rates, the temperature effect can be separated from the hydrogen effect, thereby allowing the independent monitoring of two variables by one sensor head.

As would be known by those skilled in the art, the disclosed invention is not limited to a preferred embodiment as described but may be practiced with any suitable means as is not known or known hereafter. 

1. A fiber optic sensor system for measurement of a two independent variable parameters, comprising. first means for projecting light with substantially one wave length; second means for receiving said light from said first means; said second means including third means for reflecting said light in an interferometer output signal responsive to respective first and second independent variables; fourth means for receiving said light reflected from said third means and detecting from said interferometer output signal, separate respective fringe rate signals indicative of said first and second independent variables.
 2. The system of claim 1, wherein, said third means includes means for reflecting said light in separate displaced light paths responsive to respective first and second independent variables sensed at substantially the same spatial location.
 3. The system of claim 2, wherein, said third means includes means for allowing said light in said separate displaced light paths to interfere in an interferometer output signal.
 4. The system of claim 1, wherein, said third means includes a means for reflecting said light with a first phase, a means for reflecting said light with a second phase relative to said first phase in response to said first independent variable, and a means for reflecting light with a third phase relative to said second phase in response to said second independent variable.
 5. The system of claim 4, wherein, said means for reflecting said light with a second phase relative to said first phase, includes means for transmitting a part of said light from said first means, to said means for reflecting light with a third phase.
 6. The system of claim 4, wherein, said means for reflecting said light with a first phase includes means for forming a first reflective surface in a first end in a first optical cavity, said means for reflecting light with a second phase includes means for forming a second reflective surface in a second end of said first optical cavity, and a first transmissive end of a second optical cavity, and said means for reflecting light with a third phase includes means for forming a third reflective surface in a second of said second optical cavity.
 7. The system of claim 6, wherein said means for forming said first reflective surface, said means for forming a second reflective surface, and said means forming a third reflective surface, includes means for arranging said first, second and third, reflective surfaces at spaced apart locations.
 8. The system of claim 7, wherein said means for reflecting said light include includes means for reflecting said light in light paths of separate respective optical path lengths.
 9. A three mirror optical sensor for the simultaneous measurement of independent variable at the same location, comprising, a single wavelength light source; an optical sensor, connected in a light conductive path to said light source, including a first optical cavity placed to receive light from said light source, and a second optical cavity placed to receive at least part of said light received by said first optical cavity; said first optical cavity including a first material with a response limited to a first independent variable; said second optical cavity including a second material with a response limited to said first and to a second independent variable; said first optical cavity including a first reflective surface at a first end of said first optical cavity, a second reflective surface at a second end of said first optical cavity, and at the first end of said second optical cavity, and a third reflective surface at a second end of said second optical cavity; said first second, and third reflective surfaces arranged to produce respective fringe rate signals responsive to the phase relation of said light from said light source, respectively reflected from said first, second, and third, reflective surfaces.
 10. The system of claim 9, wherein said first, second, and third, reflective surfaces are arranged displaced from each other at separate respective first, second, and third, optical path lengths.
 11. The system of claim 10, wherein said first material responsive to said first variable changes said second optical path length; and said second material responsive to said second variable changes said third optical path length.
 12. The system of claim 9, wherein said first and second optical cavities are contiguous and said second reflective surface, includes a transmissive surface for transmitting said light received by said first optical cavity to said second optical cavity.
 13. The system of claim 9, including a light conductive path for reflected light from said first, second, and third, reflective surfaces and said light conductive path is arranged to allow said fringe rate signals to interfere in a interferometer output signal.
 14. The system of claim 9, including a detector in said light conductive path arranged to separate said fringe rate signals.
 15. The system of claim 12 wherein said first and second optical cavities are arranged to simultaneously respond to said first and second independent variables at substantially the same spatial location.
 16. A method for measurement of a plurality of independent variable parameters, comprising the steps of, projecting light with substantially one wave length, receiving said light with said substantially one wave length; reflecting said light in an interferometer output signal responsive to respective first and second independent variables; receiving said interferometer output signal and detecting from said interferometer output, separate respective fringe rates indicative of said first and second independent variables.
 17. The method of claim 16, including the step of reflecting said light in separate displaced light paths responsive to respective first and second independent variables, sensed at substantially the same spatial location.
 18. The method of claim 17, including the step of allowing said light in said separate displaced light paths to interfere in an interferometer output signal.
 19. The method of claim 16, including the steps of reflecting said light with a first phase, reflecting said light with a second phase relative to said first phase in response to said first independent variable, reflecting light with a third phase relative to said second phase in response to said second independent variable, and allowing said light with said first, second and third phases, to interfere in an interferometer output signal.
 20. The method of claim 19, wherein said step of reflecting said light with a second phase relative to said first phase, includes the step of transmitting a part of said light, for said step of reflecting said with a third phase. 